This invention relates to an improvement in a fluid flowmeter, and more particularly to a fluid flowmeter using an ultrasonic transmitter and receiver apparatus and network for detecting Karman vortices generated periodically as the period of phase modulation of ultrasonic beams.
When an obstacle is placed in a fluid flow stream, Karman vortices are produced in the flow stream by the obstacle. When ultrasonic waves are transmitted through the Karman vortex, a phase difference occurs between a transmitting wave and a receiving wave. The phase difference consists of a series of phase differences A changed immediately in accordance with the period of the Karman vortices and strength thereof and the other phase differences B changed slowly in accordance with the sonic velocity in the fluid.
The phase difference is detected by a detector for phase difference using an exclusive OR circuit and a low pass filter. When the phase difference B is an integer times 180 degrees, the component of the phase difference A is not demodulated and it is necessary to control the phase difference B to a fixed value excepting 180.times.n degrees.
The main causes of changes in the phase difference B are, firstly, transmitting sound pressure against an input voltage of a ceramic microphone for the ultrasonic waves (hereinafter referred to as "MIC"), the phase of the output voltage against an input sound pressure, and the distance between the sound source and the receiver; secondly, sonic velocity in the fluid; and thirdly transmitting frequency. With regard to the first cause, it is difficult to control the phase difference B because the manufacture of the MIC and the structure of the detecting section is very complex. With regard to the second cause, it is impossible to control without change of flow speed, since the sonic velocity is changed by change in the temperature of the fluid which is detected. With regard to the third cause, due to the transmitting frequency, when the transmitting frequency is changed to reduce and control the phase difference B, it is difficult to detect stably the phase difference A, since the MIC has a sharp resonance point to operate itself. Namely, the width of the usable frequency for MIC is very narrow, and the variation of sensitiveness of the MIC due to the change of the frequency is large, and the received signals of both of the phase differences A and B become weak by the variation of sensitiveness, and subsequently, the signal-to-noise ratio of the phase modulation for the phase difference A of the received wave by the Karman vortex street deteriorates.
With regard to an air conditioning system and method thereof using the Karman vortex type fluid flowmeter of the present invention, conditioned air is fed to each chamber through branch ducts from an air conditioning apparatus placed at one place, when the air conditioning such as air cooling or heating is made to many chambers at the same time. In this system of air conditioning, a volume of air required at a respective terminal chamber is generally different in accordance with the condition of the disposition of respective branch air ducts or the air temperature in the respective chambers, and this difference is changed further in accordance with the condition of temperature which changes from time to time in the respective chambers.
In a conventional air conditioning apparatus of the above stated system, conditioned air volume which is fed from the air conditioning apparatus is controlled in respective chambers by opening or closing a damper which is respectively installed in each branch air duct, which is respectively connected to a main air duct, and the total volume of the conditioned air which is fed by an air blower is simultaneously generally controlled.
As the conventional air conditioning apparatus, it is known that there are some types of air conditioners to control air volume by opening or closing a damper in accordance with information obtained by a sensor, and such an air conditioner can use different kinds of sensors. On sensor has a semiconductor, a thermistor or a hotwire that generates heat, another sensor uses an electric generator of the propeller fan type for an air current in a duct, and still another sensor utilizes the difference of pressure compared between the dynamic pressure and the static pressure of an air current in a duct. The first sensor does not respond immediately to feed a required volume of air and does not have a wide range, and also the sensor needs temperature correction. In addition, the first sensor, which is of high cost, does not provide stable operation and is not able to respond sufficiently when the volume of air required is changed by a large margin. The second sensor, which utilizes an electric generator of the propeller fan type, must be inspected for maintenance of the electric generator and rotor and must be considered always in terms of its mechanical life, and therefore the second sensor is moderate in price, but its maintenance involves troublesome problems. The third sensor, which utilizes differences of pressures, needs revision of temperature and does not respond immediately to feed a required volume of air.